The subject matter disclosed herein relates to dynamoelectric machines. More specifically, the subject matter disclosed herein relates to rotor position determination for wound field synchronous machines.
A multi-phase alternating current (AC) dynamoelectric machine can be used as a motor or as a generator. In aircraft applications in particular, it is desirable to use such a machine for both electrical power generation and as a starter motor to promote weight and space savings on the aircraft. When used as a starter, the position of the rotor of the dynamoelectric machine must be determined in order to properly orient the stator current rotating 3-phase waveform relative to the magnetic north pole of the rotor. Orienting the stator current waveform relative to the rotor position is necessary for motor operation and proper orientation results in an optimum amount of torque being produced by the dynamoelectric machine.
Since a mechanical rotor position sensor can be costly and lacking in reliability, it is desired to determine the rotor position without the use of such a mechanical position sensor. In general, there are two categories of sensorless rotor position determinations. The first is a back EMF based method, which is relatively easy to implement and usually works well at high angular rotor velocity, but is less reliable at low rotor velocity and doesn't work at all below some threshold speed or at standstill. The second is a signal injection method, which is required below some threshold angular rotor velocity or at standstill. Systems that utilize the signal injection method, however, are subject to a rotor position error of up to 180 degrees because the system cannot recognize which of a number of possible rotor positions it has locked on to or is tracking.
A carrier injection sensorless (CIS) logic has been developed which successfully determines which of the possible rotor positions that the system is tracking providing an absolute or true rotor position. The determination of true rotor position involves two separate CIS logic blocks, the first of which interrogates the exciter stator to lock on to and track the rotor position by measuring and demodulating the current harmonics contained in the 3-phase exciter of the machine and the second which interrogates the main machine through its stator to determine which of the possible rotor positions the exciter based CIS logic block has locked on to and is tracking. This result is used to calibrate the first logic block to provide a true or absolute rotor position. The second CIS logic block has inherent north/south pole indeterminacy and determines main machine positive from negative poles by, for example, saturating the main machine rotor or using the rectification in the rotating rectifier of the machine in a secondary logic. Thus, both techniques require a two-step process whereby the primary CIS logic block locks on to a preferred position as in the exciter CIS or to a pole as in the main machine stator CIS, and the secondary CIS logic block determines which of the possible rotor positions the primary CIS has locked on to thereby providing a correction or calibration to the primary CIS logic block. The double two-step process requires significant logic complexity. Further, using rectification to provide the secondary current harmonic increases the electrical stress on the rotating rectifier diodes thereby reducing their reliability. Additionally, the main stator carrier currents required to induce saturation in the main machine may be so large at to significantly upsize the current sourcing requirements of the main machine AC power supply.